Magnetically shielded electrodynamic sound reproducer



Jan. 4, 1949. w. E'. GILMAN MAGNETICALLY SHIELDED ELECTRODYNAMIC SOUNDREPRODUCER 3 Sheets-Sheet 1 v Filed Nov. '25, 1942 J/Zorrzeys jh/fifarxZI/AJZerE 5177/2412 o a 6 0 Q o 0/ o o v 0 0 0: 0 0 0 0 0 0 Jan. 4,1949. w, GILMAN 2,458,158

MAGNETICALLY SHIELDED ELECTRODYNAMIC SOUND REPRODUCER Filed Ngv. 25,1942 3 sheet s-sheet 2 F I 5110: I F 5'. f1

116 @zfiM- 104 97 r U Patentecl Jan. 4, 1949 MAGNETICALLY smnnnnnsrlcptrno- DYNAMIC soUNn BEPRGDUCER Webster E. Gilman, Maywoed, Ill,assignor to Per-moflux Corporation, Chicago, 111., a corporation ofIllinois Application November 25, 1942, Serial No. 466,903

u ta -v 1 This invention relates to transducers and particularly totransducers of the dynamic type as contrasted with the magnetic typethereof.

In dynamic transducers as heretofore con-.

structed it has :been recognized that in order to obtain the desiredsound transducing capacity it has been necessary to resort to relativelylarge dimensions in the transducers, with the result that the weight ofsuch transducers has been excessive when compared with the outputcapacities thereof. Where size and weight were deter mining and vitalfactors, it has been customary to resort to the use of magnetic typetransducers, even though fidelity and range of frequency response havebeen sacrificed through this procedure, and accordingly it is animportant object of this invention to enable dynamic transducers to beemployed even where weight and size are determining factors in theselection of the type of transducer to be used; and further objectsrelated to the foregoing are to enable dynamic transducers to attain asubstantially flat frequency response curve and a substantially fiatimpedance curve over a wide range of frequencies such as thosefrequencies utilized in ordinary practice and particularly over therange of frequencies used in conversation; and to enable an outputcapacity to be attained in dynamic speakers which is comparable to theoutput capacity of other types of transducers of comparable size andweight.

One factor which has contributed in a large measure to the foregoingdisadvantages and limitations inherent in prior dynamic speakers hasbeen found to be in the relatively complicated manner of constructionand assembly of the elements of the magnetic circuit, and with suchprior constructions the elements of the magnetic circuit have beenenclosed. within a housing which has added objectionable size and weightto the transducers. A further object of this invention is therefore tosimplify the construction of the magnetic circuits of dynamictransducers, and an object related to the foregoing is to accomplishthis simplification in such a manner that an element of the magneticcircuit may function as the housing of the transducer. Further andrelated objects are to form the magnetic yoke of a dynamic transducerwith an annular or circular transverse cross section, to incorporatepositioning means in such an annular magnetic yoke whereby mounting ofthe outer pole piece or plate may be readily and accuratelyaccomplished, to provide a transducer wherein the operative elements arefully shielded, and to ac- (c1. ire-115.5

complish such shielding through use of the housing not only as a housingbut also as a magnetic shield.

In accordance with prior practice it has been customary to provide anindependent diaphragm support which, after assembly upon the magneticyoke, has been used to support and position the diaphragm and the voicecoil. Through the exercise of proper care and skill in the formation andassembly of the parts it has, of course, been possible to attainaccuracy of relationship of the parts in such prior transducers, buteven then, the cost and added size resulting from the use of such priordiaphragm supports have been objectionable. In view of this it is afurther object of the present invention to enable means formedintegrally with or as a part of an element of the magnetic circuit ofsuch transducers to function as a diaphragm support. According to oneaspect of my invention it is a more specific object to enable thediaphragm to be supported by means formed integrally with the magneticyoke; and a further object related to the foregoing is to form themagnetic yoke in a generally cupiike form such that the diaphragm may besupported upon the rim of the cup-like magnetic yoke while the outerpole piece or pole plate may be located Within and supported by thecup-like yoke in spaced relation to the rim thereof.

A further object of this invention is to simplify the mounting andlocation of the outer pole piece or pole plate in dynamic transducersand a more specific object is to accomplish this through the provisionof means formed in the cup-like magnetic yoke in spaced relation to thebottom wall thereof and cooperating with the separately formed outerpole piece to locate the same accurately With the relation to themagnetic yoke.

A further object of this invention is to utilize the outer pole piece orpole plate to support the diaphragm, thereby to eliminate the cost ofproduction and assembly which has heretofore been involved in theproduction and assembly of a separate diaphragm support.

A further object is to enable a relatively small permanent magnet to beused in a transducer in such a relation that an extremely 'high fluxden- Sity is produced in the magnetic gap of the transducer; and objectsrelated to the foregoing are to enable a transducer to be so constructedthat the space within the housing is efficiently utilized, thereby toproduce maximum flux density in the magnetic gap and to attain thisresult in relatively small transducers; to so mount the per- 3 manentmagnet that the magnet serves as the inner side of the magnetic gap; andto utilize permanent magnets of cylindrical form which may beeconomically produced and which serve the function usually performed bya separate inner pole piece.

It has been pointed out hereinbefore that dynamic transducers embodyingthis invention may be utilized either as microphones or speakers, andWhen a transducer is used as the speaker or earphone of a headset, manydifficulties have heretofore been encountered which have objectionablyincreased the cost of production of such speakers and have limited theusefulness of the speakers. One objection which has been found in priorspeakers of the dynamic type has been that in order to utilize suchspeakers or earphones with remotely located receiving apparatus it hasbeen necessary to utilize voice coils having a relatively high ohmicresistance, and. in order to provide such a high resistance it has beennecessary to employ numerous layers of relatively fine wire, with theconsequent difficulties in winding the coils. Accordingly it is afurther object of this invention to enable voice coils having arelatively small ohmic resistance to be employed in speakers which areto be used as parts of a headset, and a further object related to theforegoing is to accomplish this result through the incorporation of atransformer within and as a part of the speaker so that the length ofthe leads from the speaker to the receiver is rendered immaterial in apractical sense.

Speakers which are adapted for use as parts of a headset are ordinarilyarranged for cooperation with a headband of a particular size andconstruction, such headbands being in many instances provided with aclip assembly at the ends thereof adapted to be snapped into associationwith cooperating means provided on the speaker. Such clip assemblies arerelatively well standardized as to their dimensions, but in certainclasses of work it has been found that clip assemblies having twodifferent dimensions are frequently encountered, and as a result it hasbeen found that speakers or earphones are often unsuitable for use withthe headbands which are available. Therefore it is an object of thisinvention to enable speakers or earphones to be readily utilized inconnection with clip assemblies of different dimensions, and an objectrelated to the foregoing is to accomplish this in a simple andexpeditious manner. More specifically, it is an object of this inventionto utilize a cup-like magnetic yoke which has the side walls thereof soformed as to provide different engaging means of different dimensionswhereby clip assemblies of different sizes may cooperate with either oneor the other of such engaging means.

A further object is to simplify the assembly of the diaphragm and voicecoil element so that accuracy of interrelation of these elements may bereadily attained.

A further object is to enable a transducer to be rendered waterproof inthe sense that the internal electrical elements cannot be reached bywater or moisture.

A further object of this invention is to simplify the assembly of thecap plate upon the open end of the speaker housing, and an objectrelated to this is to employ interlocking means for rotatively relatingthe cap plate to the housing and to employ a simple clamping means tohold such interlocking means together.

Other and further objects of the present irivention will be apparentfrom the following description and claims and are illustrated in theaccompanying drawings which, by way of illustration, show preferredembodiments and the principles thereof and what I now consider to be thebest mode in which I have contemplated applying those principles. Otherembodiments of the invention embodying the same or equivalent principlesmay be used and structural changes may be made as desired by thoseskilled in the art without departing from the present invention and thepurview of the appended claims.

In the drawings? Fig. 1 is a front elevational view of a transducerembodying the features of Fig. 2 of the invention;

Fig. 2 is a sectional view taken substantially along the line 2-2 ofFig. 1;

Fig. 3 is a sectional view taken along the line 3-3 of Fig. 2;

Fig. 4 is a plan view looking downwardly in Figs. 1 and 2;

Fig. 5 is a view showing one face of the outer pole piece;

Fig. 6 is a sectional view of the diaphragm and voice coil assembly;

Fig. 7 is a front elevational view illustrating another alternativeembodiment of the invention;

Fig. 8 is a side elevational view of the transducer shown in Fig. 7;

Fig. 9 is a rear elevational view of the transducer shown in Fig. '7

Fig. 10 is a cross sectional view taken substantially along the linei0lll of Fig. '7;

Fig. 11 is a cross sectional-view taken substantially along the linell-ll in Fig. 10;

Fig. 12 is a fragmental sectional view taken along the line l2-l2 ofFig. 11;

Fig. 13 is a cross sectional view similar to Fig. 10 and illustrating analternative embodiment of the invention;

Fig. 14 is a cross sectional view similar to Fig. 10 and illustrating analternative embodiment of the invention;

Fig. 15 is a sectional view taken along the l5-l5 of Fig. 14; and

Fig. 16 is a sectional view taken along the line |6I6 of Fig. 15.

In the form of the invention illustrated in Figs. 1 to 6 inclusive ofthe drawings, the invention is embodied in a transducer l0 adapted foruse either as a microphone or as a speaker, this particular transducerbeing intended and adapted primarily for use in portable radiobroadcasting and receiving units so as to avoid duplication oftransducers therein. In accordance with the present invention thetransducer in has its elements so formed and related that the magneticcircuit is formed in part by a cup-like member I I made from a ferrousmetal such as steel which constitutes the magnetic yoke and embodies abottom wall I2 and an annular side wall l3. Thus the cup-like magneticyoke ll may function as ahousing for the operative elements of thetransducer. The side wall I3 is tapered from its juncture with thebottom wall l2 to a somewhat larger diameter at the outer end thereof,and the bordering edge of the outer end of the annular side wall I3 hasan outwardly projecting annular flange l4 formed integrally therewithand parallel to the bottom wall 12 to facilitate mounting of thetransducer III in the unit in which it is to be employed. At a pointspaced from therear or bottom wall l2 line and from the flange I4, theside wall I3 has an annular rabbeted groove I8 formed therein to receivethe outer peripheral edge of a soft iron disc I9 which serves as theouter pole piece or pole plate of the transducer, and it will beobserved that when the pole piece I9 is mounted in the rabbeted grooveI8, the pole piece I 9 is spaced a substantial distance from the bottomWall I2 of the yoke to afford space within which a permanent magnet 26and a soft iron inner pole piece 2! may be disposed. It will also beobserved that when the pole piece I9 is thus mounted it is spaced fromthe end flange I4, thereby to afford space within which a diaphragm 25may be disposed.

The permanent magnet 20 is in the present case frusto-conical in formand has its larger end seated against the inner face of the bottom wallI2 of the cup-like magnetic yoke, thereby to establish an eflicientmagnetic contact with the magnetic yoke when the magnet 2!! is drawnfirmly against the bottom wall I2. The permanent magnet 26 has a recess26 extending therethrough which may be in the form of a lateral slot orin the form of a bore, and a screw 21 is extended through a relativelylarge opening 28A formed in the bottom wall I2 so as to project throughthe recess or bore 26 and into threaded engagement with the inner polepiece 2 I. The inner pole piece ZI has a relatively wide and thickflange 28 formed on one end of a cylindrical body 29 so that when thescrew 2'! is tightened a relatively large area contact is establishedbetween the small end of the magnet 20 and the flange 28 of the innerpole piece 2i. The cylindrical portion 29 of the inner pole piece 2I isarranged to extend into a central circular opening 30 formed in theouter pole piece or pole plate I9 centrally thereof, and thus an annularmagnetic gap is afiorded between the inner and outer pole pieces, and avoice coil structure 3| carried on the diaphragm 25 is arranged toextend into this annular magnetic gap.

In order to assure accuracy of formation and dimensions of the magneticgap, the pole piece I9 has a locating plate 35 formed from a nonmagneticmaterial such as brass and secured in position on the inner face of thepole piece I9 by rivets 36. The locating plate 35 has an offset portion31 spaced from the plane of that face of the pole piece I9 upon which itis mounted. The portion 37 of the locating plate 35 has a circularopening 38 formed therein arranged to snugly embrace the periphery ofthe cylindrical portion 29 of the inner pole piece 2|, and in practicethe openings 30 and 38 formed respectively in the pole plate I9 and thelocating member 35 are simultaneously cut to their final size b asuitable trimming tool so as to insure concentric location of the twoopenings 30 and 38.

In the assembly of the transducer ID, the permanent magnet 20 and theinner pole piece 2| are first placed in position within the cup-likemagnetic yoke II and the screw 21, bearing a lock washer iii, isinserted through the opening 38 in the wall I2 and through the recess orbore 25 of the permanent magnet 20 and into threaded engagement with theinner pole piece 2|. The screw 21 is not tightened at this time,however, for it will be recalled that the locating ring 35 is reliedupon to accurately position the pole piece 2i. The outer pole piece orplate I9 is then placed in position in the groove I8 with the eatingplate 35 engaged with the cylindrical portion 29 of the pole piece 2|,and the pole piece I9 is fixed into the rabbeted groove I 8 with a pressnu While such a press fit in many iristances affords thedesiredmechanical and magnetic connection between the pole piece I9 andthe annular side wall I3 of the magnetic yoke, I prefer to swage themetal of the wall I3 as at 42 in order to firmly connect the side wallI3 with the pole piece I9. Such a swaging operation may be convenientlyperformed along theedges of air relief slots or openings 43 which areformed in the side Wall I3 intermediate the flange I4 and the rabbetedgroove I8, and to facilitate this swaging operation, the edges 43' ofthe openings 43 are preferably spaced slightly outwardly from the outerface of the pole plate I9. Thus after the outer pole piece I9 has beensecured permanently in position within the cup-like magnetic yoke II andin the accurately determined location afforded by the rabbeted grooveI8, the pole piece 2! will also be located accurately through the actionof the locating plate 35. Thus after this has been done, the screw 21may be tightened to produce the desired magnetic connection between thepole piece 2| and the wall l2 of the magnetic yoke and the opposite endsof the magnet 20;

The diaphragm 25 is preferably provided with an auxiliary supportingstructure operable to damp the vibratory movement of the diaphragm andthereby maintain the voice coil 3I in its optimum relation to themagnetic gap, and this end is attained in the transducer of Figs. 1 to 6through the provision of a flexible suspension member 44 mounted on theoutermost face of the pole plate I9 and engaging the diaphragm justoutwardly of the voice coil structure. Thus the suspension member 44 isgenerally disc-like in form and is made from a flexible sheet materialsuch as paper impregnated with a material such as a thermo-settingplastic. The suspension member 44 has a central opening therein definedby an integral cylindrical portion 45 which is adapted to surround thevoice coil structure 3| in spaced relation thereto so that the free edgeof the cylindrical portion 45 may engage the inner face of the diaphragm25. At the other end edge of the cylindrical portion 45 the suspensionmember extends outtvardlyto afford a flexing portion 44 which preferablyincludes a plurality of concentric corrugations, and at the outer edgean angularly disposed portion extends'from the portion 44' to merge withan outwardly projecting flat mounting flange 46. The suspension member44 is mounted on the pole plate I9 with the cylindrical portion 45concentric with the annular magnetic gap 30, and this may beaccomplished through the use of a thermo-plastic adhesive which isapplied to the flange 46 thereof, and when the suspension member is tobe secured in position, the flange 46 thereof is pressed against thepole plate I9 under considerable heat and pressure so that thethermo-plastic material bonds the flange 45 to the pole plate l 9.

After the foregoing assembly operations have been performed, thediaphragm 25 may be placed in position and the leads from the voice coilmay be connected to the respective terminals #8 and 49 which are carriedon an insulating'terminal plate 50 secured on the outside of the wallI3. It should be observed that while the elements of the terminal blockmight be put in place on the wall .13 prior to the assembly of theelements of the magnetic circuit, I prefer to do this surface of thehousing H may be freely used for centering and supportingpurposesduringassembly of these parts. 49 are secured on the terminal plate 50by rivets so that connecting arms 52 of the terminals 48 and 49 projectbeyond one edge of the terminal plate; and the terminal plate 50 ispositioned on the housing I l with an interposed layer 53 of insulatingmaterial, and is secured in place by screws 54 extended through theterminal plate 5|] and threaded into the wall of the housing II.

In preparation for its mountingin position in the housing I i, thediaphragm has a pair of small metal rivets 55 secured therein as shownin Figs. 1, 2 and 6 and the ends of the voice coil, are connectedthereto, as by solder, and at the same time flexible woven lead wires 56are soldered-to the respective rivets 55 on the rear face of thediaphragm 25. The lead wires 56 are extended loosely and in a radialrelation toward the open lugs 43 which are adjacent to the terminalblock 50 and are drawn through openings in opposite ends of a relativelystiff insulating sheet 51 which is secured by arivet 58 on the innerface of the wall [3 so as to extend at its opposite ends partiallyacross two of the openings 43. The'openings in the insulating sheet 51through which the lead wires 56 are extended are so positioned as tomaintain the lead wires out of contact with the metal of the housing II,and preferably a drop of solder is placedon the wires 56 outside of theinsulating sheet 5'! to prevent undue looseness of the lead wiresbetween the sheet 51 and the rivets 55. Short insulating sleeves 69 arethen placed on the extended ends of the wires 56 which are then threadedthrough outwardly extending openings in the arms 52 of the terminals 48and 59 and are soldered thereto at 52' on the outer surfaces of thearms; The lead wire 56 which is thus connected to the terminal 48 isextended as at 56' and is soldered to the rivet 58 to afford a groundconnection inaccordance with usual practice.

The diaphragm and the cylindrical Voice coil support, in this embodimentof the invention, are separately formed as shown in Fig. 6, and aresecured together by means such as adhesive, and it will be observed thatthediaphragm 25.has a central opening therein which in the finishedtransducer is closed by a cap 65 as shown in Figs. 2 and 4. When theleads 56 are being-connected as just described the diaphragm 25 and thevoice coil structure are located loosely in position, and after theconnections between the wires 56 and the terminals 48 and 49 have beencompleted a plurality of relatively thin shims made from paper or likematerial are placed in position between the cylindrical portion 29 ofthe inner pole piece and the inner surface of the voice coil form,thereby to accurately locate and center the voice coil structure in theannularmagnetic gap The bordering edge ofthe diaphragm 25 is thensecured in position in a rabbeted annular groove formed in the flangel4, and this may be accomplished through utilization of a thermo-plasticadhesive, the procedure being the same as that. described hereinbeforein connection :with the mountin of the suspension member 44. The shimsare then removed and the cap is se.-- cured in place by adhesive tocomplete the assembly of the transducer.- I 1 In the transducer 10 whichis thus provided the space between the diaphragm 25 and the suspensionmember 44 is freely vented by the openings 43, while the space behindthe suspension Thus the terminals 48 and- EDI member 44 is vented intothe relatively large V01?" ume chamber afforded behind the pole plate 19by a plurality of openings 51 formed in the pole plate l9. In someinstances the damping of the diaphragm 25 may be improved by restrictionof the air flow through the openings 61, and this may be accomplished bygluing or otherwise for the principal elements of'the speaker and alsoserves as a magnetic yoke for the magnetic circuit. lhe housing H isformed as by suitable stamping and drawing operations to provide abottom wall l2 and a generally annular or cylindrical side wall l3 whichis flanged outwardly at Mat its outer end. In this embodiment of theinvention the outer pole piece or pole plate 79 is mounted on the flangeM rather than directly uponthe side wall of the housing, and is securedin place thereon by rivets extended through openings appropriatelyspaced about the flange i l and the mounting edge of the pole plate ill.The pole plate :9 is dished inwardly as shown in Figs. 10 andlZ so thatthe central portion thereof in which a circular opening 8! is formed isspaced inwardly of the housing H, and through this arrangement space isafforded for the mounting of a diaphragm 82 between the pole plate 19and a cap plate 83 which closes a substantial portion'of the open end ofthe housing H and affords an open central area 85 through which soundenergy may pass.

In the assembly of the pole plate 19 and the cap plate 83-011 thehousing ii, the flange M is provided with a plurality of openings Bil ofa size adapted to receive the rivets 8B, but such rivets 8d are employedonly in alternate ones of the openings Bfiflthe others of such openingsbeing engaged by projections 85 formed at appropriate points about themounting edge of the pole plate incorporate a foraminous iron or steelplate 89 through substantially the entire area of the cap plate. In themolding operation the openings 90 throughout the central area 85 aremaintained free of the plastic material, but throughout the balance ofthe area of the plate 89 the plastic extends through the openings 90 soas to be interlocked with the plate 89. The outer face of the plate 89is preferably covered and concealed by the plastic material except inthe central area 85; and the plastic material is formed to provideclearance sockets ill for the heads of the rivets 8B and to-provideprojections 88 for engagement with the sockets 8'! in the pole plate l9.

After the diaphragm 82 has been secured in place by adhesive or the likeon the outer face of the mounting edge of the pole plate '79, the capplate may be put in position, suitable notches In this embodiment of thei preferably being formed in the edge of the diaphragm to aifordclearance for the rivet heads and the projections 88. To secure theseparts together, a clamping ring 93 is put in position with one flange 94engaging the outer face of the cap plate 83 and another flange 95 isformed at the other edge of the ring, as by spinning, to engage the rearface of the flange I4. Thus the housing II is effectually closed and theinternal elements of the speaker are shielded by the plate I9 which isincorporated in the cap plate 83. These internal elements of the speakerare, of course, mounted within the housing II prior to the final closureof the housing as aforesaid, and these elements include meanscooperating with the cup-like magnetic yoke afforded by the housing IIand with the pole plate I9 to afford an annular magnetic gap withinwhich a voice coil structure 96 carried by the diaphragm 82 may operate.

Thus, as shown in Figs. 10, 11 and 12, a cylindrical permanent magnet 91is disposed in the housing ll substantially coaxial therewith, and 2fscrew 98 is extended through a longitudinal bore 99 in the magnet 91 andinto threaded engagement with a stationary nut provided by a plate I80which is Welded to the bottom wall I2 of the housing. The head of thescrew 98 is seated in the counterbored central opening of a washer [IIIand the screw is initially left fairly loose until the proper centeredposition of the magnet 97 has been determined. In accordance with thepresent invention as illustrated in Figs. 10, 11 and 12 the outer end ofthe magnet 91 is utilized as the central pole piece of the magneticcircuit, and hence this end of the magnet extends into the opening 8Iformed in the outer pole piece or pole plate I9. To insure an accuratelyformed magnetic gap the pole plate I9 has a locating plate I93 securedby rivets I04 on the rear face thereof, this locating plate being formedfrom a non-magnetic material such as brass to afford an offset annularportion I95 arranged to snugly embrace the magnet 91. Hence when thepole plate '19 is secured in place on the housing H the locating plateI03 acts to center the magnet 91 in the opening ill of the pole plateI9, and thus an accurately formed magnetic gap is afforded. The screw 98may then be tightened to produce an effective magnetic path between thebottom wall of the housing and the adjacent end of the magnet ill.

The voice coil structure 96 is preferably formed with its cylindricalvoice coil form molded integrally with the flexing portion of thediaphragm as described in detail in my Patent No. 2,440,439, issuedApril 2'7, 1948, and this voice coil form includes an annular externalshoulder IIlI facing away from the diaphragm 82 as to serve as a guidefor locating the voice coil on the form. This external shoulder I9! isformed by offsetting the material of the voice coil form, and hence acorresponding internal shoulder is also formed so as to face toward thediaphragm. Thus when the screw 93 has been tightened, a diaphragm buttonI98 having a cylindrical flange I99 thereon may be mounted in closingrelation to the outer end of the voice coil form merely by inserting theflange I 99 into the form until it engages the internal shoulder, inwhich position it is secured by means such as adhesive. The diaphragmbutton I538 in this instance constitutes a part of the loading portionof the diaphragm structure and is slightly domed to impart strengththereto.

With this arrangement the head of the screw 98 and the washer IIlI areshaped to provide a domed form generally complemental to the internalform of the diaphragm button I I18 and spaced uniformly therefrom. Theopen central area of the foraminous plate 89 is also slightly domed asshown in Fig. 10 to conform with'the shape of the diaphragm button I08.

The ends of the voice coil, formed by wires I I 9, are preferably ledthrough the diaphragm 82, near the voice coil form, as shown in Fig. 12,and on diametrically opposite sides of the form and are led outwardly ingenerally opposite directions along the forward or outer face of thediaphragm, to points substantially aligned with openings II I formed inthe pole plate I9. At these points the wires IIO are led back throughthe diaphragm. The portions of the wires III) which are thus located onthe forward or outer face of the diaphragm are preferably covered by awater proof surface coating. The ends of the wires III] are led throughthe openings II I where they are soldered to the free ends of connectingclips M2, the other ends of the clips II2 being secured to the rear faceof the pole plate I9 by rivets H3.

The voice coil lead wires III] must, of course, be electricallyassociated with a receiver or other source of signal current, and thisis accomplished by means including a terminal block I I5 mounted withinthe housing II. As shown in Figs. 10 and 11, the terminal block I I5 ismolded from insulating material to include a neck portion I I6 whichprojects outwardly from the housing II through a complemental slot II Iformed in the side wall '3 thereof, and a screw IIB extending throughthe wall I2 and a bore in the terminal b ock H5 is threaded into one armof an angle bracket I I I, which is in turn he d in place by a rivet 8'which passes through the s de wall I3 and the other arm of the anglebracket. This serves to hol the terminal block firmly in position.

The terminal block H5 has two parallel bores or sockets IZEI formedtherein and opening outwardly through the neck portion H6, and. thesesockets are arranged to intersect with and extendthrou h metallic seeves I2I which are mo ded in the terminal b ock in spaced and insulatedrelation to each other. The sockets IZE! are of a size adapted to snuglyreceive metall c connectorthe outside of the back wall I2 of the casingII.

The tips of the connecting wires may thus be held in position in thesockets I20 and in electrical association with the respective metalsleeves I2I; and to provide for connection of the metal sleeves I2I tothe voice coil, each sleeve I 2! has a metallic ear or clip I23extending therefrom into an accessible position within the housing II.

It has been pointed out hereinbefore that earphones or speakers areoften used in remote locations with respect to the receiver with whichthey are associated, and in the past it has been the practice to employhigh resistance voice coils in such earphones in order to produce aproperly matched impedance in the plate circuit of the amplifier. Thisprior practice has resulted in anexceedingly high cost and large weightand size in dynamic speakers, but in accordance with the presentinvention these objections are avoided through the provision ofatransformer I25 included within the housing II and employed as anelectrical coupling between the metal terminals I21 and. the lead wiresl in of the voice coil. Thus, as shown in Figs. 10 and ll, thetransformer SE5 embodies mounting bracket are which is fixed upon thepole plate 74 9 by a rivet I2? or the like. The transformer I25 is sodisposed on the pole plate IQ that when the pole plate is put inposition in the housing I! the transformer will be disposed within thehousing a position diametrically opposite from the terminal block IE5.Prior to the mounting of the pole plate IS in the housing the lead wires28 from the secondary winding of the transformer E25 are connected tothe respective connector clips H2, there being insulating sleevessurrounding the intermediate portions of the wires I28, and the endsthereof,

being connected to their respective clips H2 as by soldering. The leadwires I29 from the ends of the primary winding of the transformer I25are relatively long, and prior to the mounting of the pole plate '19 inthe housing ll, insulating sleeves are placed on the wires i253 and theextreme ends of the wires I29 are connected by soldering to therespective clips E23 of the terminals IZ'I. Thus when the assembly ofthe parts has been completed the speaker of this invention. embodiesmeans for matching the impedance of the receiver with which it is to beused, and hence. the voice coil may be of a relatively small size andresistance and the other elements of the speaker. may be correspondinglyreduced in size. the present speaker is adapted for use under all of theconditions encountered in practice. and particularly Where long leadsare to be interposed between the receiver and the speaker.

When speakers of the kind shown in Figs. 7 to 12 are in use they areordinarily mounted on the ends of a headband of conventionalconstruction, and such headbands embody clip assemblies matched with theheadbands available, thereby causing objectionable delay in setting upor repairing a communication system. However, in accordance with thisinvention, the casing II is so formed as to afford two different sets ofbearing sockets spaced different distances apart so as to enabledifferent styles or sizes of clip assemblies to be employed therewith.In accomplishing this desirable result the wall 13 of the housing II isofiset inwardly at diametrically opposite points to form substantiallyparallel wall sections 1.3. These wall sections I3 are disposed insymmetrically related positions on ops posite sides of the opening II"!and the terminal block H5, and as shown, in Fig. 8, the wall sections 13extend from the bottom wall 12 toward the flange M for a distanceslightly less than one-half the distance between the bottom wall and theflange M. Thus the wall I3 is of cylindrical. form between the. ledge113av and the flange I4. Av pair of aligned bearing sockets Hill areformed in the wall sections I3 substantially centrally thereof forcooperation with clip assemblies of one predetermined dimension orstandard, while similar aligned bearing sockets I3I are formed in theareas of the wall 13 sub- Thus:

I vention has been embodied [ill stantially midway between the ledges13a and the flange I4 for cooperation with clip assemblies of adifierent predetermined dimension or standard. Thus the speaker III ofthis invention may be readily put to use with headbands of two differentstandards to thereby facilitate set-up or repair oi radio and likecommunication systems.

As pointed out hereinbefore, the total weight of a transducer is ofter acontrolling factor in determining whether or not it is suitable for usein a particular situation, and while the, embodiments illustrated inFigs. 1 to 12 have maintained the high and desirable performancecharacteristics usually associated with dynamic transclucers Whilematerially reducing the weight of such transducers, an even greaterweight reduction may be attained through the use of the embodiment ofthe invention illustrated in Fig. 13. In this form, the inin atransducer III which is most of its structural characteristics issimilar to the transducer I0 hereinbefore described, and in allinstances where such correspondence exists the same reference numeralswith a prime suffix have been applied in Fig. 13.

Thus the transducer 10' has a housing II which is cup-shaped and isflanged at its open end as at I4. In this form of the invention thehousing II also serves as the magnetic yoke of. the device, and a poleplate 19A is mounted on the open end of the housing as by rivets 80. Thepole plate 19A is dished inwardly in the same general manner as in thetransducer I0, thereby to locate the central opening 8i thereof aboutone end of the permanent magnet 97 and in an inwardly spaced relation tothe cap plate 83. In this embodiment, however, the pole plate ISA isformed so that it is relatively thin near its outer edge, as indicatedat I33, and gradually increases in thickness toward the central opening84. With this arrangement an effective magnetic path is afiorded fromthe outer edge or flange I34 to the magnetic gap, and yet a substantialreduction in material and in weight is efiected. It will be observedthat the flange I34 of the housing may be minimized, a relatively wideand flat nut I36 is utilized to receive the end of the screw 98'. Thenut I36 has a lug I31 thereon adapted to engage a socket formed by ahalf-punch I38 in the bottom wall of the housing, thereby to hold thenut against rotation as the screw 98' is tightened. The nut I36 is soformed as to bear firmly against the outer face of the bottom wall I2over a wide area, thereby to afford an eihcient magnetic path from themagnet 91. through the Wall, I2 and to the nut I36, and because of itsthickness the nut I35 serves to afford ample cross sectional area in themagnetic path near the central axis of the housing- The nut I36 is.tapered or rounded to a thin outer edge, as indicated at I39, but suchtaper is gradual so that the nut extends outwardly to point. where theefiectiv-e cross sectional area, of the. wall 12. is suflicient. toprovide an efilcient magnetic path.

With this, arrangement it is in many instances possible. to somewhatreduce the Wall; thickness of the housing 'II', and as a result, theweight of the transducer as a whole may be minimized while maintainingefiicient operation and per-v formance thereof.

In locations adjacent to magnetically sensitive apparatus such asmagnetic direction finding or indicating equipment. it is. esentialthatv stray magnetic fields adjacent to such sensitive apppae. ratus beeliminated or at least minimized. With the transducers illustrated inFigs. and, 13 it will be. evident that the housing 'II or 'II'. servesas the primary path for the magnetic flux: so that stray magnetic fieldswill be present about such transducers. Where such stray magnetic fieldsare objectionable as in the above cited instances, the present inventionmay be embodied in the alternative form shown in Figs. 14 to 16 whichretains the general form and size of the trans.- ducers of Figs. 10 and13;. but is so constructed and arranged that stray magnetic fields areavoided. Thus the transducer 10a of Figs. 14 to 16 has a generallycup-shaped housing 'I.Ia which is flanged at Ma at its outer end and isformed with sockets 130a. and I3Ia so as to be adapted for mounting inclip assemblies of different sizes or designs in the same manner as thetransducers of Figs. 10 and 13.

In this alternative form of the invention the housing II (1 serves as apath for conducting the magnetic flux to the annular magnetic gap of thetransducer, but in this form of the invention this function of thehousing Ila is accomplished. in such a manner that the. housing acts asa shield rather than as a primary part of the magnetic circuit, andmeans is provided within the housing I I a to afford a primary magneticcircuit. To this end a magnetic yoke I40 is formed from a relativelyheavy strip of soft iron or other magnetically permeable material tafford a bottom member I4I, two side members I42 disposed at rightangles to the bottom member MI, and a top member I43 which is providedby bending the ends of the side members I42 inwardly toward each other.These ends may be joined together as by welding, and the two ends joinedare punched to aiiord an opening BIa which forms the outer side of themagnetic gap. of the transducer, and hence the member I43 constitutesthe outer pole piece in this embodiment of the invention. On its innerface the top member I43 has a locating member I 05a. made fromnonmagnetic material and held in place by rivets at I04a.

member I4I of the magnetic yoke. I40, and a screw 98a extended throughan axial bore 99a. in.

the magnet is threaded into the bottom member I4I to hold the magnet91a. in position.

The magnetic yoke I40 is supported in position within the housing 'IIaby a mountingplate 19a which has a central opening therein somewhatlarger than the opening Bla so that the central portion of the plate 19amay be secured tov the outer face of the top member I43 in a centralrelation to the. opening 8Ia, and such a connection is in the presentinstance afforded by the rivets I04a. The mounting plate 19a is dishedinwardly so that when its outer. flange or edge portion 19b is placedagainst the outer face of the flange 14a, the magnetic gap afforded bythe opening 8Ia will be disposeda substantial distance inwardly of theopen endof. the housing Thus a permanent magnet 91a of cylindrical formmay be extended through the. locating member IflEa and into contact withthe bottom Ha. A. diaphragm 8.2a of a. generally cone.--

shaped form has a voice coil structure 96a supported thereon in suchposition that it may be centered Within the magnetic gap afiordedbebetween the opening 8| a and the magnet 91a. The outer edge of thediaphragm 82a is disposed in abutment with. the outer face of themounting flange I911, and a magnetic shield in the form of a metal plate8511; has its peripheral edge 85b disposed against the outer mountingflange portion ofthe diaphragm 82a. The plate 850., the diaphra-gmBZa;and the mounting plate 19a. are held in position on the flange 14a ofthe housing by a plurality of rivets 80a which are extended throughtheseveral flanges, and in the present instance a fibre gasket I45 isalso secured in place on the rear face of the flange 14a by means of therivets a. After the shield or plate a has been secured in position asjust described the outer cap plate 83a; may be placed in position overthe plate 85a, suitable interlocking means bein provided so thatopenings a. in the cap plate 83a may be accurately aligned withcorresponding openings 90b in the shield plate 85a. After the cap plate8341 has been placed in position, it is secured in this position by anannular clamping ring 93a. In the present instance the shield plate 85ais offset in its central portion as indicated at I41, and a sheet offabric I48 is secured as by adhesive to the plate 85a in the spaceafforded by the offset I41. Through this arrangement the reproductioncharacteristics of the present transducer may be governed to produce thedesired frequency response characteristics.

The transducer of Figs. 14 to 16 has a terminal block II5a which issubstantially similar to the terminal blockshown in Fig. 10 and which issecured in position by means such as a rivet M. In connecting theterminals I2Ia of the terminal block to the coil of the voice coilstructure 96, a transformer I25a is utilized in substantially the samemanner as in the embodimentshown in; Fig. 10, and wires I 290. areextended from therespective terminalsv I2la to the opposite terminals ofthe primary of the transformer I25a. The transformer [25a is supportedon the plate 19a by means including rivets IZIa. Wires I-28aare extendedfrom the secondary of the transformer to rivets I49 which are mounted bymeans ofinsulating -sleeves I50 in the mounting plate 19a as shown in-Fig. 14, and wires IIlla are extended from the rivets I49 through thediaphragm 82a and to the opposite ends of the voice coil in the samemanner as in Fig. 12.

It will be.- observed that the bottom member [4.1 of the. ma netic yokeI40 is supported out of contact with the wall- 'I 2aof; the housing, anda similar spaced relation is afforded between the side members I412 and:the side walls of the housing. Since the sockets. I3IJ'a are formed inthe oifset portions of the side wall, clearance slots I52: are formedin; the members I42 so that the inwardly projecting bosses, formed inthe wall 73a. when the sockets tim are stamped, do not contact thesidemembers I42. clear that any stray magnetic fields about themagnetic-yoke I46 wil'l" pass to the walls 12a. or 13a of the housing Haand will be conducted to the mounting platev 79a.- and thence to themember M3: which constitutes the outer pole piece and" defines the outerside of the magnetic gap. Hence. the casing; Mar in this, embodiment ofthe invention serves asa path formagnetic flux, but

' since the primary magnetic path afforded by the Thus it will beyokeI40 provides the best and most efiective path to the magnetic gap, theflux density in the housing Ha is maintained relatively small at alltimes, such flux density in the housing Ila being so small thatobjectionable stray fields outside the housing are avoided. Where thewalls of the housing are relatively thin as in this embodiment of theinvention, the bottom Wall 1211 may be strengthened by the formation ofan embossed panel 12b therein.

In securing the desired frequency response in the transducer the backloading of the diaphragm. 82a is preferably reduced through theprovision of a plurality of openings 19c in the mounting plate 79a, thisarrangement being such as to enable substantially the entire interior ofthe housing to function as a back chamber. Further improvement of thefrequency response is also attained by forming the diaphragm 82a with anannular bead or corrugation 82b in the outermost portion of the freearea of the diaphragm, and with this bead it is found that the flexingof the diaphragm isrelatively free under all conditions. The mounting ofthe textile member I48 between the cap plate and the shield 85a alsocontributes to the attainment of the desired frequency response, for byselection of the fabric it is possible to eliminate or reduceundesirable components of the voice energy.

It will be evident that the embodiments of this invention illustrated inFigs. 7 to 16, as thus described, are waterproof in character since thediaphragms are of waterproof construction and are arranged to cooperatewith the housing in excluding water from the internal electricalelements, and in furtherance of this purpose a suitable lacquer may beapplied within the housing about the terminal block and at other pointswhere leakage might occur. When, however, the transducers are employedfor some uses, as for example, in aviation work, it is desirable toprovide for equalization of the pressure in the back chamber, and wherethis is desirable and where absolute protection against moisture is notvital, this may be accomplished by providing relatively small pin holesin the diaphragm adjacent to the edge thereof.

Under other circumstances, where the water proof character of thetransducer is to be preserved. the present invention enables the desiredequalization of the pressure in the back chamber to be attainedthrough-means which'prevents entry of water. While such means might beafforded in any of the embodiments shown in Figs. '7 to 16, it is shownonly in Fig. 14 where an opening I54 is formed in the panel 12b of thehousing and a resilient sheet or membrane I55 of rubber or the like issecured on the inner face of the bottom wall 12a so as to span therecess afforded by the panel. The sheet I55 is secured to the bottomwall 12a by adhesive so as to exclude water from the electrical elementswithin the housing, and yet when the pressure within; the housingexceeds that outside the housing, the resilient sheet I55 may bestretched into the recess afiorded by the panel 121), thereby to reducethe pressure in the back chamber to correspond with the external airpressure which is applied to the sheet I55 through the opening I54.

Where the back chamber is thus completely closed or sealed, the airconfined within the back chamber is of course compressed as'an'incidentto the operation of thediaphragm, and because of the elastic propertiesof "such air an undesirable variation' in the frequency response. of thetransducer may result at some frequencies. but in accordance with thisinvention, such an undesired variation is avoided by the provision ofmeans operable to dissipate the energy of the air which is thuscompressed within the back chamber. As shown in Fig. 14, such means areafforded by providing a fibrous filler material in a substantial portionof the back chamber, and while this construction may be employed in anyof the illustrated embodiments, it is herein shown in but one of suchembodiments. Thus, as shown in Fig. 14, the rear portion of the housingis filled with a fibrous material I60 such as kapok or wood fiber. ablycompressed into blocks of the shaperequired to fit about the elements ofthe magnetic circuit and the transformer so that the preformed block offibrous material may be readily placed in position, but if desired thematerial I60 may be packed in place bit by bit while the mounting plate1% is held in a slightly separated relation to the open end ofthe'housing. Where the material is thus put in place bit by bit it hasbeen found desirable to place a protective'shield of textile fabricacross the openings 190, Fig. 16, or the openings III, Fig. 11, for bysuch an expedient the filler material I60 is maintained out of contactwith the diaphragm at all times. It will be understood that for clarityof disclosure, the fibrous filler material I 60 is shown in but a smallportion of the back chamber in Fig. 14, but in practice the fillermaterial is placed in position in all or a substantial portion of theback chamber, and is compressed or packed relatively firmly so as toplace a high resistive load on'the back of the diaphragm and have anefficient dissipating action with respect to the energy of the air assuch air is compressed. When the back chamber has been filled to arelatively great extent with wood fiber it has been found that thepresent transducers of Figs. 7 to 16 have a frequency response whichconforms more closely to the desired fiat response curve than any priortransducer, and such close conformity has beenfound to extend throughthe range of frequencies from below one hundred cycles to above fourthousand cycles. Moreover, it has been found that with the transducersof this invention, the variation from the normal frequency responsewhich is usually experienced at high altitudes is reduced to asubstantial extent, thereby to produce highly improved operation in highaltitude aviation uses. Thus these transducers of Figs. '7 to 16 insuremaximum intelligibility of transmission under all noise conditions, andsuch intelligibility materially exceeds that attained by priortransducers.

From the foregoing description it will be evident that the presentinvention enables dynamic transducers to attain .the desired transducingcapacity while maintaining the size and weight thereof at the minimum,and hence the transducers of this invention may be utilized where suchconsiderations are vital factors in determining what type of transduceris to be'selected. The use of a cup-shaped housing which also functionsas a magnetic yoke contributes in a large measure to the attainment ofthis desirable result, and it will be observed in this connection thatthe cup-shaped housing affords a magnetic path of large cross sectionalarea so as to increasethe' effectiveness of the magnet in produc ingmaximum flux density in the magnetic gap. Moreover, the use of acylindrical magnet extending into the outer. pole piece and serving asThe fibrous material ISO is prefer-- 17 the .inner pole piece enablesthe maximum .of magnetic force to be supplied in a transducer of algiven'size. These novel features, of course, enable maximum magneticflux to be attained at the magnetic gap to thereby contribute to theefficiency of the transducer as a Whole, and par ticularly it has beenfound that the high flux density attained in the magnetic gap of thetransducers of this invention enables the voice coil to be reducedradically insize and resistance to thereby correspondingly contribute tothe desirable characteristics of the transducers of this invention. Thecup-shaped yoke and housing employed in the present transducers is sorelated to the form and location of the outer pole piece or pole platethat the yoke may also function as the primary supporting means for thediaphragm, and hence the transducers are simplified as to constructionand assembly.

When the present invention is embodied in a speaker or earphone thearrangement is such that the cup-shaped housing provides means formounting the earphone on headbands having different sizes of clipassemblies, and hence the usefulness of the earphones is greatlyincreased. In this embodiment of the invention the efiiciency of thecup-shaped magnetic yoke is such that the magnet may be relativelysmall, and hence the location of the coupling transformer Within thehousing of the earphone is attained in accordance with this inventionWithout sacrifice of compactness in the structure asa Whole.

In the illustrated embodiments of the invention it Will be evident thatunusual simplicity of structure and assembly has been attained so thatthe transducers may be produced Wi'thgreat economy of time and material.Moreover, it will beseen that the construction and arrangement of thediaphragm and voice coil assembly contributes materially to the economyof the present construction, particularly in the mounting of thediaphragm button on the diaphragm in the form shown in Fig. 10, for itWill be clear that the diaphragm button may be quickly located in itsfinal position merely by insertion of the flange of understood thatthese are capable of Variation and modification and I therefore do notwish to be limited to the precise details set forth, but desire to availmyself of such changes and alterations as fall Within the purview of thefollowing claims.

I claim:

1. In a transducer, means includin a permanent magnet and an outer polepiece of magnetically permeable material arranged to cooperate to definean annular magnetic gap adjacent to one end of the magnet, a diaphragmhaving a voice coil structure disposed in said magnetic gap, a cup-likehousing formed from magnetically permeable material and disposed aboutsaid magnet in such a relation that said annular magnetic gap is locatedinwardly of the open end of said cuplike housing, said pole piece beingsecured to said cup-like housing to afiord .a magnetic path between thepole piece and the housing, and means completing a magnetic circuit fromthe other end of said magnet to said outer pole piece to enable at leasta portion of the magnetic flux to flow through said housing to said polepiece.

2. In a transducer, a cup-like housing formed from magneticallypermeable material and having a bottom wall and a generally annular sidewail,:a mounting plate extended across .the open end of said housing andsecured to the side Wall of said housing to provide a magnetic pathfromsaid mounting plate to said side Wall, a ma netic yoke formed frommagnetically permeable material to provide a bottom member, ide membersand atop member apertured to constitute said top member as an outer polepiece, said outer pole piece being permanently secured to said mounting.rnember to support said magnetic yoke Within said housing with saidbottom member and side members thereof in spaced relation to saidbottom- Wall and sidewall respectively of said housing, a permanentmagnet mounted on said magnetic yoke With one end of the magnet in fluxtransmitting contact with said bottom member of the yoke and'vvith theother end disposed Within said aperture in said pole piece to define anannular magnetic gap, and a diaphragm having a voice coil structuredisposed withinsaid annularmagnetic gap and having "the outer edges ofthe diaphragm mounted on the outer edges of said mounting plate.

3. In a transducer, a magnetic yoke formed from magnetically permeablematerial to afford a bottom member, two side member and a top member.having a central aperture therein, a perma nent magnet secured to saidbottom member and extended-into said aperture in said top member toafiord'an -annular magnetic gap, a mounting plate formed frommagnetically permeable .material and secured .to said top member, acup-like housing formed from magnetically permeable material "andhavin'gsaid mounting plate secured across the :open 'end thereof to supportsaid magnetic yoke within said housing in spaced relation to theWalls'thereof, said mounting plate :being secured to said housing at theopen end thereof to afford a magnetic path between said plate and saidhousing whereby stray magnetic flux passing from said yoke to saidhousing is transmitted through said mounting plate to said top member ofsaid magnetic yoke.

4. In a transducer, a cup-like housing formed from magneticallypermeable material and having a bottom Wall and a generally annular sideWall, a mounting plate extended across the open end of said housing andsecured to the side wall of said housing to provide a magnetic path fromsaid mounting plate to said side wall, a magnetic yoke formed frommagnetically permeable material to provide a bottom member, side membersand a top member apertured to constitute said top member as an outerpole piece, said outer pole piece being permanently secured to saidmounting member to support said magnetic yoke Within said housing Withsaid bottom member and side members thereof in spaced relation to saidbottom Wall and side wall of said housing, a permanent magnet mounted onsaid magnetic yoke with one end of the magnet in flux transmittingcontact with said bottom member of the yoke and with the other enddisposed within said aperture in said pole piece to define an annularmagnetic gap, a diaphragm having a voice coil structure disposed withinsaid annular magnetic gap and having the outer edges of the diaphragmmounted on the outer edges of said mounting plate, a shield plate ofmagnetically permeable material positioned across and in spaced relationto the movable parts of said diaphragm, a cap plate of insulatingmaterial positioned against the outer face of said shield plate, saidcap plate and said shield plate having a plurality of correspondinglypositioned perforations therein through which voice energy may pass intocontact with said diaphragm,means for locating said cap plate and saidshield plate in such a relation that said perforations are in registry,and means for holding said cap plate and said shield plate in positionon said housing.

5. In a transducer, a magnetic yoke formed from magnetically permeablematerial to afiord a bottom member, two side members and a top memberhaving a central aperture therein, a permanent magnet secured to saidbottom member and extended into said aperture in said top member toafford an annular magnetic gap, a mounting plate formed frommagnetically permeable material and secured to said top member, acuplike housing formed from magnetically permeable material and havingsaid mounting plate secured across the open end thereof to support saidmagnetic yoke within said housing in spaced relation to the wallsthereof, said mounting plate being secured to said housing at the openend thereof to afford a magnetic path between said plate and saidhousing whereby stray magnetic flux passing from said yoke to saidhousing is transmitted through said mounting plate to said top member ofsaid magnetic yoke, a diaphragm having a voice coil structure disposedwithin said annular magnetic gap and having the outer edges of thediaphragm mounted on the outer edges of said mounting plate, a shieldplate of magnetically permeable material positioned across saiddiaphragm, a cap plate of insulating material positioned against theouter face of said shield plate, said cap plate and said shield platehaving a plurality of correspondingly positioned perforations thereinthrough which voice energy may pass into contact with said diaphragm,means for 20 locating said cap plate and said shield plate in such arelation that said perforations are in registry, a sheet of fabricdisposed between said shield plate and said cap plate, and meansforholding said cap plate and said shield plate in position on saidhousing.

WEBSTER. E. G'ILMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 966,247 Schwarz Aug. 2, 19121,115,519 Edwards Nov, 3, 1914 1,546,567 Childress July 21, 19251,586,391 Timmons May 25, 1926 1,702,434 Hanna Feb. 19, 1929 1,792,300Hanna Feb. 10, 1931 1,901,898 Clarke Mar. 29, 1933 1,960,449 Midgley May29, 1934 1,961,665 Holly June 5, 1934 1,987,413 Olson Jan. 8, 19352,010,537 Emmert Aug. 6, 1935 2,020,211 Quam Nov. 5, 1935 2,029,282Serge Jan. 28, 1936 2,037,185 Thuras Apr, 14, 1936 2,058,555 Betts, eta1 Oct. 27, 1936 2,085,721 Warnke June 29, 1937 2,119,272 Engholm May31, 1938 2,155,474 Creager Apr. 25, 1939 2,162,270 Mott June 13, 19392,180,615 Serge Nov. 21, 1939 2,202,886 Abrahams June 4, 1940 2,216,961Samial Oct. 8,1940 2,293,078 Proctor Aug. 18, 1942

